One of the major limitations in therapy today is the toxicity or side effect of drugs. The maximum tolerated dose of a drug can thus be a hindrance for many therapies including those for cancer. There is a need for conjugates that specifically bind to a disease specific target and release the drug at the disease site. Targeted drug delivery using antibodies, for example, has been investigated extensively (R. V. J. Chari, “Targeted cancer therapy: conferring specificity to cytotoxic drugs,” Accounts of chemical research, vol. 41, no. 1, pp. 98-107, January 2008). There are currently two antibody drug conjugates (ADCs) on the market for cancer therapy namely KADCYLA™ and ADCETRIS™. They both use a cytotoxic drug covalently conjugated to an antibody through a cleavable linker.
Protein A is a small bacterial protein that has an affinity for the Fc region of IgG class of antibodies (T. Moks, L. Abrahmsén, B. Nilsson, U. Hellman, J. Sjöquist, and M. Uhlén, “Staphylococcal protein A consists of five IgG-binding domains.,” European journal of biochemistry/FEBS, vol. 156, no. 3, pp. 637-43, May 1986). The domain that non-covalently binds to the Fc region is already known and is used for monoclonal antibody chromatographic purification extensively (S. Hober, K. Nord, and M. Linhult, “Protein A chromatography for antibody purification.,” Journal of chromatography. B, Analytical technologies in the biomedical and life sciences, vol. 848, no. 1, pp. 40-7, March 2007). Several protein A mimetics and small molecules have been explored in the past to replace protein A chromatography such as triazines, 4-mercaptoethyl pyridine (4-MEP), peptides (VS. Kabir, “Immunoglobulin purification by affinity chromatography using protein A mimetic ligands prepared by combinatorial chemical synthesis.,” Immunological investigations, vol. 31, no. 3-4, pp. 263-78, 2002) etc. They all have affinity for the Fc region similar to protein A but each offering some advantage over the conventional protein A (S. Ghose, B. Hubbard, and S. M. Cramer, “Evaluation and comparison of alternatives to Protein A chromatography Mimetic and hydrophobic charge induction chromatographic stationary phases.,” Journal of chromatography. A, vol. 1122, no. 1-2, pp. 144-52, July 2006). 4-MEP and triazines have also been investigated for treatment of autoimmune diseases (J. Ren, L. Jia, L. Xu, X. Lin, Z. Pi, and J. Xie, “Removal of autoantibodies by 4-mercaptoethylpyridine-based adsorbent.,” Journal of chromatography. B, Analytical technologies in the biomedical and life sciences, vol. 877, no. 11-12, pp. 1200-4, April 2009 and B. Zacharie, S. D. Abbott, J.-F. Bienvenu, A. D. Cameron, J. Cloutier, J.-S. Duceppe, A. Ezzitouni, D. Fortin, K. Houde, C. Lauzon, N. Moreau, V. Perron, N. Wilb, M. Asselin, A. Doucet, M.-E. Fafard, D. Gaudreau, B. Grouix, F. Sarra-Bournet, N. St-Amant, L. Gagnon, and C. L. Penney, “2,4,6-Trisubstituted Triazines As Protein a Mimetics for the Treatment of Autoimmune Diseases.,” Journal of medicinal chemistry, vol. 53, no. 3, pp. 1138-45, February 2010).
Some of the peptides, owing to their affinity for the antibody, have also been used for targeting nanoparticles for drug delivery (H. J. Kang, Y. J. Kang, Y.-M. Lee, H.-H. Shin, S. J. Chung, and S. Kang, “Developing an antibody-binding protein cage as a molecular recognition drug modular nanoplatform.,” Biomaterials, vol. 33, no. 21, pp. 5423-30, July 2012 and US Patent Publication No. 2011/0312877, and European Patent Application No. EP 2 093 287 A1). The targeting antibody directs the nanoparticles in close proximity to the target site wherein the nanoparticle can deliver its cargo. Non-covalent interactions between the linker and drug have been employed to conserve the activity of the drug U.S. Pat. No. 5,420,105. Biotin labelling of biomolecules has been reported for affinity-based diagnostics (US 2001/0023288 A1) wherein the affinity of biotin towards streptadivin is employed.
The binding site on IgG of the different affinity molecules has been found to be different from each other. Binding site for 4-MEP on IgG Fc has been computationally determined previously (Lin, D.-Q., Tong, H., Wang, H. & Yao, S. Molecular insight into the ligand-IgG interactions for 4-mercaptoethyl-pyridine based hydrophobic charge-induction chromatography. J. Phys. Chem. B 116, 1393-400 (2012).) Binding site of traizine on IgG Fc has also been similarly determined computationally (Branco, R. J. F., Dias, A. M. G. C. & Roque, A. C. A. Understanding the molecular recognition between antibody fragments and protein A biomimetic ligand. J. Chromatogr. A 1244, 106-15 (2012).).